The polymeric aromatic carbamates (polyurethanes), such as the diphenylmethane dicarbamates and related higher homologs, the polymethylene polyphenyl carbamates, and especially the diethyl esters, are important products, particularly for use in the preparation of the commercially valuable diphenylmethane diisocyanates and mixtures of diisocyanates and polyisocyanates by the thermal decomposition of such polymeric aromatic carbamates in a suitable solvent as shown, for example, in Rosenthal et al, U.S. Pat. Nos. 3,962,302 and 3,919,279.
Prior art processes have been proposed for the preparation of polymeric aromatic carbamates (polyurethanes) by the acid condensation of N-aryl carbamic esters with carbonyl compounds such as formaldehyde, para-formaldehyde and a formaldehyde forming compound such as trioxane and ketones. Klauke et al, U.S. Pat. No. 2,946,768, describes the condensation of aryl carbamic esters with carbonyl compounds such as aldehydes and ketones with a dilute mineral acid. Shawl U.S. Pat. Nos. 4,162,362 and 4,202,986 disclose the condensation of N-aryl carbamic acid esters with formaldehyde, paraformaldehyde or trioxane in the presence of an organic sulfonic or halogenated organic sulfonic acid having an acid concentration of at least 75 percent and a Lewis acid intercalated in graphite respectively.
In processes employing dilute aqueous acid systems, the carbonyl compound such as formaldehyde tends to react at the nitrogen of the carbamate to produce along with some desired di- and polyurethanes, varying amounts, i.e., generally between 15 percent and 50 percent by weight of undesirable N-(alkoxycarbonyl)phenylaminomethylphenyl compounds which include the di- and polyphenyl compounds, which compounds referred to as "N-benzyl" compounds are fully described in U.S. Pat. No. 4,146,727. Attempts to prepare diisocyanates and polyisocyanates or to otherwise use the mixture containing the polyurethanes and such amounts of the undesirable compounds, which compounds cannot be converted to isocyanates by pyrolysis, present many problems since there is no known method for separating the polyurethanes from the N-(alkoxycarbonyl)phenylaminomethylphenyl impurities. While U.S. Pat. No. 4,146,727 describes a method for converting the "N-benzyl" compounds employing certain strength acids it is advantageous to essentially avoid the formation of such impurities as will occur with dilute acid condensation systems or dilute acid condensation systems resulting from the water of condensation formed in the reaction.
Azeotropic distillation of water and azeotropic distillation during chemical processes are known. U.S. Pat. No. 3,000,859, describes the production of thermoplastics resins by the acid condensation of poly benzenoid aromatic hydrocarbons and aldehydes. U.S. Pat. No. 3,809,724 describes a process for the preparation and recovery of ethylene glycol by azeotropic distillation with a water-immiscible agent which forms a minimum-boiling azeotrope with the ethylene glycol.